GPS radio occultation signals are produced when a GPS radio signal passes through a portion of the atmosphere and the signal experiences phase and amplitude modulations. A receiver positioned such that the portion of the atmosphere is between the GPS satellite and the receiver may record the occulted GPS radio signal. The phase and amplitude measurements can be processed to determine information about the atmosphere, such as bending angle and refractivity. Other parameters of the atmosphere, such as air density, temperature, moisture, pressure and ionospheric electron density may be derived from the phase and amplitude measurements.
The receivers are typically on board Low Earth Orbiting (LEO) satellites, but can also be on board satellites in different orbits or on other platforms such as airplanes, balloons, etc. The GPS satellites orbit the earth at an approximate altitude of 20,200 km while LEO satellites orbit the earth at an approximate altitude of 300-1,000 km. The relative motion of the GPS and LEO satellites results in frequent occultations of the GPS satellite as seen from LEO satellite.
Typically, Phase-Locked Loop (PLL) tracking is used to lock on to GPS signals. However, during occultations, fluctuations in both phase and amplitude may be much greater than can be tracked by PLL signal processing. To track the occulted radio signal under such conditions, open-loop tracking of the occulted radio signal may be used. Open-loop tracking is basically the raw sampling of the down converted complex signal. In open-loop tracking there is no feedback between the phase model used for the down conversion and the received signal. An advantage of open-loop tracking is that it is not susceptible to the complicated structure of the input signal and the input signal can be reliably recorded without distortion. One disadvantage of open-loop tracking is the low signal-to-noise ratio due to aliasing of the noise into the sampling frequency domain. To overcome this disadvantage, in the open-loop mode the radio occultation signal is down-converted with the phase model calculated without a feedback from the signal and then low-pass filtered. Such a system is described in U.S. Pat. No. 6,731,906 to Sokolovskiy and Rocken, the entirety of which is included herein by reference.
During PLL signal processing, the 90-degree phase Navigation Data Modulation (NDM) of the GPS radio signal may be removed in-receiver so that the effect of the occultation on the carrier phase can be determined. However, during open loop tracking without feedback, it is not generally possible to remove the NDM in-receiver. It has to be removed in the post-processing. The post-processing of the recorded open-loop radio occultation signal in order to determine its phase and amplitude described in U.S. Pat. No. 6,731,906 does not describe removal of the NDM.